Machine vision devices are used in many manufacturing industries to identify either work piece defects or to identify known and expected patterns in work pieces. Generally, these vision systems include an illumination system or light source, a camera and a controller. The light source and camera are arranged adjacent a manufacturing line station. The source shines light on a surface of the work piece at the station and the camera is arranged such that a field of view (FOV) includes the illuminated surface of the work piece.
The controller is linked to the camera and causes the camera to periodically take exposures of the surface. The controller then compares each exposure or some characteristic thereof to ideal work piece characteristics (i.e., an ideal exposure or expected characteristic) and, based thereon, performs some function. Where unintended patterns or defects are identified, the controller may either flag the defective work piece for quality control purposes or, in the alternative, may log the unintended pattern or activate an indicator so that the overall manufacturing process can be adjusted thereby reducing the number of similar defects in the future.
One particularly useful area in which vision systems are used for both quality control and process adjustment purposes is in industries that manufacture planar web materials such as paper, metallic foils, etc., that are wound on rolls for delivery to end users. In web manufacturing applications, as in other vision system applications, a light source is used to illuminate a portion of the material and a camera is positioned to obtain exposures of at least a segment of the illuminated portion. However, because of the nature of the materials being examined and how the materials are typically stored, vision systems used with web based applications have additional requirements.
First, as a material web is being moved about a facility and prior to winding on a roller or roller, the web typically vibrates such that obtaining any type of meaningful image of the web material is difficult at best. Similarly, during transport, web material characteristics change as a function of system parameter settings and operation. For instance, the tension between adjacent web transport rolls tends to stretch the web to varying degrees during transport. For this reason, even if vibrations were minimal at some point during the manufacturing process, images at most points along web transport would vary as a function of system parameters such that the images would be imperfect for determining the characteristics of the end product.
For these reasons, in the case of imaging web products, vision systems typically are used to examine the end web product as the product wraps about a roller just prior to the web being wound on an end spool. By inspecting the web on the roller adjacent an end spool, the inspection point is rendered relatively stable and therefore vibrations are minimized.
In addition, examining the web on a roller instead of on the end spool avoids problems with attributing sensed defects to wound web sections. For instance, assume a small but persistent defect occurs along the length of a web material being wound on an end spool and that the material is being examined as it accumulates on the end spool. In this case, during a first winding, the degree of defect reflects reality and is relatively minimal. However, during a second winding, assuming the defect remains aligned at the same point along the web width, the defects from the first two windings accumulate and the defect sensed during the second winding appears greater than it is in reality. This cumulative process continues and the sensed degree of defect continues to grow. Complicating matters further, once defects have accumulated there is no way to attribute portions of the cumulative defect to web segments.
Second, a camera set and light source having a special field of view is typically used to examine roller supported web material. To this end, the term “cross direction” or first direction will be used hereinafter to refer to the dimension of a web supported material along the length of the roller (i.e., along the width of a web). In order to simplify this explanation the direction along a web surface perpendicular to the cross direction and tangent to the radius of curvature of an outwardly facing segment of the web at a given point will be referred to as “machine direction” or second direction. In order to get consistent inspection over 100% of the product surface, image data is collected along a line parallel to the roller cross direction (i.e., the roller length dimension).
For this reason, in order to reduce the variance among exposures as material is inspected, in web-roller based applications, the light source is typically designed to concentrate a bright light into a narrow line along a cross direction (i.e., width) segment of the material and a camera is designed and positioned to have a linear field of view that collects image data from the illuminated segment. The camera can be oriented to view a direct reflection of the illuminated segment (i.e., be placed directly in the path of reflected light rays) or direct transmission of the illuminated segment (i.e. be placed on a side of the web opposite the light so light transmitted through the web is detected by the camera). Images generated with a camera that views a direct reflection or direct transmission are generally referred to as “bright field” images. In the alternative, the camera can be oriented “off-axis” relative to the reflected or transmitted light rays resulting in a so-called “dark field” image.
Third, in most cases web type material defects are topographical along the surface of the material including bumps, indentations, etc. Such defects can be illuminated best where light is directed at an angle with respect to the surface of the web so that the defects cause shadows across the web material surface.
One way to provide light at an angle with respect to the web material surface is to provide a line source adjacent the outward facing web material surface and direct light rays at an angle with respect to the machine direction. While this solution facilitates observation of web defects along the material direction, unfortunately, this solution fails to facilitate observation of cross direction defects along the web width. Web generating processes typically vary more across web width than along web length and therefore, while this solution may enable identification of some web material defects, many defects are unobservable in this manner.
One way to provide light at an angle with respect to the cross direction dimension of a material web has been to provide a bright side light on the side of a roller (i.e., at a roller end) that shines light along rays that form a small “incidence angle” with the web cross direction dimension. While this solution works well at points near the side light source, unfortunately, along the web material width, light intensity tapers off and therefore imaging results along the cross direction are inconsistent. In addition, in the case of side light illumination, the incident angle of a light and the outward facing web material surface may vary over the entire inspected surface. Variables such as differing light intensity along web material width and changing incident angles complicate vision system tasks and therefore should be avoided whenever possible.
Therefore, a need exists for an illumination system that can provide even intensity light at a uniform angle with respect to the cross direction of a web or planar material for imaging purposes so that defects across the cross direction can be highlighted and identified.